Process for composite free layer in CPP GMR or TMR device

ABSTRACT

The conventional free layer in a CPP GMR or TMR read head has been replaced by a tri-layer laminate comprising Co rich CoFe, moderately Fe rich NiFe, and heavily Fe rich NiFe. The result is an improved device that has a higher MR ratio than prior art devices, while still maintaining free layer softness and acceptable magnetostriction. A process for manufacturing the device is described.

This is a divisional application of U.S. patent application Ser. No.10/999,826, filed on Nov. 30, 2004, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to the general field of CPP GMR/TMR read headswith particular reference to the free layer sub-structure.

BACKGROUND OF THE INVENTION

The principle governing the operation of most magnetic read heads is thechange of resistivity of certain materials in the presence of a magneticfield (magneto-resistance or MR). Magneto-resistance can besignificantly increased by means of a structure known as a spin valvewhere the resistance increase (known as Giant Magneto-Resistance or GMR)derives from the fact that electrons in a magnetized solid are subjectto significantly less scattering by the lattice when their ownmagnetization vectors (due to spin) are parallel (as opposed toanti-parallel) to the direction of magnetization of their environment.

The key elements of a spin valve are illustrated in FIG. 1. They areseed layer 11 (lying on lower conductive lead 10) on which isantiferromagnetic layer 12 whose purpose is to act as a pinning agentfor a magnetically pinned layer. The latter is a syntheticantiferromagnet formed by sandwiching antiferromagnetic coupling layer14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).

Next is a non-magnetic spacer layer 16 on which is low coercivity (free)ferromagnetic layer 17. A contacting layer such as lead 18 lies atopfree layer 17. Not shown, but generally present, is a capping layerbetween 17 and 18. When free layer 17 is exposed to an external magneticfield, the direction of its magnetization is free to rotate according tothe direction of the external field. After the external field isremoved, the magnetization of the free layer will stay at a direction,which is dictated by the minimum energy state, determined by thecrystalline and shape anisotropy, current field, coupling field anddemagnetization field.

If the direction of the pinned field is parallel to the free layer,electrons passing between the free and pinned layers suffer lessscattering. Thus, the resistance in this state is lower. If, however,the magnetization of the pinned layer is anti-parallel to that of thefree layer, electrons moving from one layer into the other will suffermore scattering so the resistance of the structure will increase. Thechange in resistance of a spin valve is typically 8-20%.

Earlier GMR devices were designed so as to measure the resistance of thefree layer for current flowing parallel to its two surfaces. However, asthe quest for ever greater densities has progressed, devices thatmeasure current flowing perpendicular to the plane (CPP), as exemplifiedin FIG. 1, have also emerged. CPP GMR heads are considered to bepromising candidates for the over 100 Gb/in² recording density domain(see references 1-3 below).

A related effect to the GMR phenomenon described above is tunnelingmagnetic resistance (TMR) in which the layer that separates the free andpinned layers is a non-magnetic insulator, such as alumina or silica.Its thickness needs to be such that it will transmit a significanttunneling current.

An MTJ (magnetic tunnel junction) is readily visualized by substitutinga very thin dielectric layer for spacer layer 16 described above for theGMR device. The principle governing the operation of the MTJ in magneticread sensors is the change of resistivity of the tunnel junction betweentwo ferromagnetic layers when it is subjected to a bit field frommagnetic media. When the magnetizations of the pinned and free layersare in opposite directions, the tunneling resistance increases due to areduction in the tunneling probability. The change of resistance istypically 40%, which is much larger than for GMR devices.

If CoFe/FeNi (Fe rich NiFe) is used as the free layer in both TMR andCPP sensors they will have a 20 to 30% GMR ratio gain compared to thetypical CoFe/NiFe (Ni rich NiFe such as permalloy) free layer. Howeverthere is some concern regarding the free layer magnetic softness becauseCoFe/FeNi deposited on top of alumina or copper will not be as soft asCoFe/NiFe. The invention discloses how the improved ratio can beachieved without an associated decrease in the magnetic softness of thefree layer as well as retaining a low positive magnetostriction.

A routine search of the prior art was performed with the followingreferences of interest being found:

U.S. Pat. Nos. 6,436,526 and 6,778,427 (Odagawa et at) disclose a NiCoFealloy for the free layer. A Ni rich film is preferred because theresistance of the Ni-rich film is much higher than that of a Fe-richfilm. U.S. Pat. No. 6,661,626 (Gill) shows a free layer comprising FeO,CoFe, and NiFe.

Additionally, reference is made to HT04-015 (application Ser. No.10,854,651 filed May 26, 2004) which deals with a similar problemtowards which the present invention takes a different approach.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a CPP GMR magnetic read head having an improved MRratio as well as a free layer that is magnetically soft.

A further object of at least one embodiment of the present invention toprovide a TMR magnetic read head having an improved MR ratio as well asa free layer that is magnetically soft.

Another object of at least one embodiment of the present invention hasbeen to provide a process for manufacturing said read heads.

Still another object of at least one embodiment of the present inventionhas been that said process be compatible with existing processes for themanufacture of GMR and TMR devices.

These objects have been achieved by replacing the conventional freelayer by a tri-layer laminate comprising Co rich CoFe, moderately Ferich NiFe, and heavily Fe rich NiFe. The result is an improved CPP GMRdevice that has a higher CPP GMR ratio than prior art devices, whilestill maintaining free layer softness and acceptable magnetostriction. Aprocess for manufacturing the device is also described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a GMR/TMR stack of the prior art which has a conventionalfree layer.

FIG. 2 shows a GMR or TMR stack having a modified free layer accordingto the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a CPP spin valve structure having a free layer such as Fe₅₀Co, higherspin polarization ferromagnetic material can improve CPP GMRsignificantly. It is also known that Fe rich NiFe has higher spinpolarization. The present invention shows how this may be applied to theNiFe component of the free layer to improve the CPP GMR.

Instead of using single composition NiFe layer both Fe rich (FeNiy) andNi rich (NizFe) layers are combined as part of the free layer of a TMRor CPP sensor. By adjusting FeNi and NiFe thickness and composition itbecomes possible to improve the magnetic softness of the free layer andstill maintain the higher MR ratio.

Because of the magnetostriction consideration, FeNi-(30-34 at %) and80-83 at %-NiFe were selected as the most suitable candidates.

Referring now to FIG. 2, we provide a description of the process of thepresent invention. In the course of this description, the structure ofthe present invention will also become apparent.

The process begins with the formation of lower lead 10 onto which isdeposited seed layer 11 followed by pinning layer 12. Layer 12 comprisesa suitable antiferromagnetic material such as IrMn and it is depositedto a thickness between 45 and 80 Angstroms. Layer 13 (AP2), the first ofthe two antiparallel layers that will form the synthetic AFM pinnedlayer, is then deposited onto layer 12. This is followed by layer of AFMcoupling material 14 and then AP1 layer is deposited thereon.

Next, non-magnetic spacer layer 16 is deposited on AP1 layer 15. For theTMR design, layer 16 would be a dielectric layer of a material such asalumina between about 5 and 10 Angstroms thick.

Now follows a key feature of the invention by which the free layer 17 ais made up of three sub-layers 21-23:

Layer 21, which is deposited onto layer 16, is a layer of Ni_(x)Fe_(y),where x is between about 75 and 90, with 82.5 being preferred and y isbetween about 10 and 25, with 17.5 being preferred. It is between about5 and 100 Angstroms thick with about 20 Angstroms being preferred. Layer22, which is deposited onto layer 21, is a layer of Ni_(w)Fe_(z), wherew is between about 28 and 40, with 34 being preferred, and z is betweenabout 60 and 72, with 66 being preferred. It is between about 5 and 100Angstroms thick with about 10 Angstroms being preferred. Note that it isalso possible to reverse the order in which layers 21 and 22 aredeposited.

The third sub-layer making up the free layer is layer 23. This is alayer of Co_(x)Fe_(y), where x is between about 70 and 100, with 90being preferred, and y is up to with 10 being preferred. It is betweenabout 5 and 100 Angstroms thick with about 10 Angstroms being preferred.

The process concludes with the deposition of capping layer 24 followedby upper lead layer 18, the completed structure being now ready to serveas a CPP GMR/TMR read head having a MR ratio of at least 7%.

Confirmatory Results

To confirm the effectiveness of the invention, a number of structureswere formed and then evaluated as CPP GMR readers:

All structures comprised Seed/Pin/Cu/Free/Cu/Cap (with full CPP Stackbeing subjected to a final anneal). The free layers were then varied asshown in TABLE I:

No. Free Hce Hch Hce/Hch λ 1 12 Å 90 at %-CoFe/35 Å 82.5 at %-NiFe 6.090.71 8.62 2.76E−07 2 12 Å 90 at %-CoFe/35 Å 34 at %-NiFe 15.02 0.8218.32 2.00E−06 3 I0 Å 90 at %-CoFe/15 Å 34 at %-NiFe/25 Å 82.5 at %-NiFe 6.02 0.60 10.03 2.08E−06 4 I0 Å 90 at %-CoFe/10 Å 34 at %-NiFe/28Å 82.5 at %-NiFe 5.65 0.90 6.28 1.21E−06 5 I0 Å 90 at %-CoFe/25 Å 82.5at %-NiFe/15 Å 34 at %-NiFe 5.67 0.40 14.18 2.08E−06 6 I0 Å 90 at%-CoFe/20 Å 82.5 at %-NiFe/25 Å 34 at %-NiFe 6.19 0.42 14.74 2.02E−06

Table I CPP Free Layer Magnetic Properties Measured by B-H Looper Hce isFree Layer Easy Axis Coercivity; Hch is Free Layer Hard Axis Coercivity;λ is the Magnetostriction Constant

TABLE I is experimental data from a CPP free layer study. Sample #1 isour typical CPP free layer. Sample #2 gives a much higher MR ratio dueto the Fe rich NiFe free layer. However the Hce and Hch of sample #2 ismore than double compared to sample #1. The results for samples #3 to #6clearly show that, by adjusting free layer CoFex, FeNiy and NizFethickness and composition, Hce and Hch can be improved several timescompared to sample #2. This will be true whether or not we deposit FeNiyor NizFe first. Magnetostriction is also adjustable into the desirablerange (from 0 to −2E-6) by free layer thickness and composition. Asalready noted, this general concept could also be applied to the designof TMR sensor free layers. Also as previously noted, higher CPP GMRratio are associated with this free layer design.

1. A process to manufacture a TMR read head, having a free layer,comprising: depositing, in unbroken succession on a lower lead layer, aseed layer and a pinning layer; on said pinning layer, depositing apinned layer; depositing a dielectric tunneling layer on said pinnedlayer; on said dielectric tunneling layer, depositing a layer ofNi_(x)Fe_(y), where x is between 75 and 90 and y is between 10 and 25;on said Ni_(x)Fe_(y) layer, depositing a layer of Ni_(x)Fe_(z), where wis between 28 and 40 and z is between 60 and 72; on said Ni_(w)Fe_(z)layer, depositing a layer of Co_(x)Fe_(y), where x is between 70 and 100and y is between 0 and 30, said two NiFe layers and said CoFe layertogether constituting the free layer; and on said free layer, depositinga capping layer and then an upper lead layer, thereby forming said CPPGMR read head.
 2. The process described in claim 1 wherein saidNi_(x)Fe_(y) layer is deposited to a thickness between about 5 and 100Angstroms.
 3. The process described in claim 1 wherein said Ni_(w)Fe_(z)layer is deposited to a thickness between about 5 and 100 Angstroms. 4.The process described in claim 1 wherein said Co_(x)Fe_(y) layer isdeposited to a thickness between about 5 and 100 Angstroms.
 5. Theprocess described in claim 1 wherein said pinned layer is a syntheticantiferromagnet that comprises oppositely magnetized ferromagneticlayers separated by an antiferromagnetic coupling layer.
 6. A process tomanufacture a TMR read head having a free layer, comprising: depositing,in unbroken succession on a lower lead layer, a seed layer and a pinninglayer; on said pinning layer, depositing a pinned layer; depositing adielectric tunneling layer on said pinned layer; on said dielectrictunneling layer, depositing a layer of Ni_(x)Fe_(y), where x is between28 and 40 and y is between 60 and 72; on said Ni_(x)Fe_(y) layer,depositing a layer of Ni_(w)Fe_(z), where w is between 75 and 90 and zis between 10 and 25; on said Ni_(x)Fe_(y) layer, depositing a layer ofCo_(x)Fe_(y), where x is between 70 and 100 and y is between 0 and 30,said two NiFe layers and said CoFe layer together constituting the freelayer; and on said free layer, depositing a capping layer and then anupper lead layer, thereby forming said CPP GMR read head.
 7. The processdescribed in claim 6 wherein said Ni_(x)Fe_(y) layer is deposited to athickness between about 5 and 100 Angstroms.
 8. The process described inclaim 6 wherein said Ni_(w)Fe_(z) layer is deposited to a thicknessbetween about 5 and 100 Angstroms.
 9. The process described in claim 6wherein said Co_(x)Fe_(y) layer is deposited to a thickness betweenabout 5 and 100 Angstroms.